The US does attract plenty of scientists from other parts of the world, including China and Russia.

Leave it to the US to make their first priority at the end of WWII to save the Nazis. If you helped murder a million children and you wanted to die of old age a respected multimillionaire in your bed surrounded by your great grandchildren:

1) Get out of Poland, now.

2) Get out of anywhere to the east of Poland.

3) Get to American-controlled Germany. Register with the Americans, and say you were a low level bureaucrat. You will be immediately released, possibly with a stipend, and rearrested a couple of years later. Go to trial, be sentenced to life or death. You will be released before 1954. There will be a network of people you killed children with to help you get back on your feet, leaders of government and industry. Your fate after this relies on how much your peers respected your ability to kill. If you were good at it, and never complained, the sky is the limit.